1. Field of the Invention
The present invention relates to a filtering device and related wireless communication receiver, and more particular, to a filtering device and related wireless communication receiver for reducing circuit layout area and increasing adjustability.
2. Description of the Prior Art
In a broadcast system, a superheterodyne receiver is the most widespread use receiver, which can execute carrier frequency adjustment (namely select a channel), filtering, and amplifying. In the superheterodyne receiver, signal is received by an antenna, and performed amplifying, RF (radio-frequency) filtering, IF (intermediate frequency) transformation, and finally, via one or more IF amplifying and filtering processes, transformed to a base frequency band for succeeding demodulation. Transforming RF to IF is always influenced by image frequency interference, and may cause some problems.
Please refer to FIG. 1, which is a schematic diagram of a superheterodyne receiver 10 according to the prior art. The superheterodyne receiver 10 includes an antenna 100, a low noise amplifier 102, an image reject filter 104, a mixer 106, a local oscillator 108, an IF low pass filter 110, and an IF amplifier 112. Below is a summary of an operation method of the superheterodyne receiver 10. An RF signal VRF1 is received by the antenna 100, and is amplified to an RF signal VRF2 via the low noise amplifier 102. Then, the image reject filter 104 filters out image frequency signals of the RF signal VRF2, to generate a filtered RF signal VFRF. Finally, the filtered RF signal VFRF is transformed to an IF band through the mixer 106 to output IF signal VIF via filtering of the IF low pass filter 110 and amplifying of the IF amplifier 112. The image reject filter 104 is used for removing interference of the image frequency. A cause of the image frequency is: two input frequencies |fLO±fIF| both become a frequency fIF through the mixer 106. The frequency fLO is an oscillating-signal frequency of the local oscillator 108, and the frequency fIF is a frequency of the IF signal VIF. Therefore, in the superheterodyne receiver 10, when a signal with spectrum corresponding to sides of a local oscillating signal goes through the mixer 106, the signals enter the same spectrum, and form an interference signal which lowers a signal to interference ratio, influences a desired received signal, and affects a receiving efficiency of the superheterodyne receiver 10. For solving the problem of image frequency interference, the most common method is to add a band pass filter in front of the mixer 106, i.e., the image reject filter 104, for filtering out the interference signal before entering the mixer 106, so as to lower the interference.
There are many methods for realizing the image reject filter 104 according to the prior art, for example, hairpin band pass filter, parallel-coupled line filter, etc. Please refer to FIG. 2, which is a schematic diagram of a hairpin band pass filter 20 according to the prior art. The hairpin band pass filter 20 is a transverse symmetry structure, which includes micro-strip ports IO_a and IO_b, and resonators RSN_1˜RSN_n. The micro-strip ports IO_a and IO_b connect to a front-stage and a rear-stage circuit for receiving and outputting signals. A total length of each of the resonators RSN_1˜RSN_n is half of a wavelength corresponding to a desired received signal, and the number “n” of the resonators RSN_1˜RSN_n represents an order of the hairpin band pass filter 20. Therefore, a designer can vary the number “n” according to different demands.
Therefore, the hairpin band pass filter 20 can achieve a proper image frequency rejection effect via adjusting a total length, an amount, a width, etc of each of the resonators. However, in the hairpin band pass filter 20, the resonators occupy a large circuit board area and increase cost because each of the resonators is bend-shaped (or hairpin-shaped). Moreover, an ability of the hairpin band pass filter 20 for restraining noise is weak around sides of a pass band. In other words, when noise closes to an RF band, the noise may enter the circuit, and cause interference. In this situation, the prior art utilizes a matched network of a micro-strip line, such as an open stub with a total length equal to a quarter of wavelength, to generate a rejection band for restraining noise.
Please refer to FIG. 3, which is a schematic diagram of a micro-strip line open stub structure 30. The micro-strip line open stub structure 30 extends an open stub 300 having an open terminal in a transmission path (from input port PT_i to output port PT_o), to generate a rejection bandwidth. However, the rejection bandwidth generated by the open stub 300 is about 30%, and an effect of reducing bandwidth is poor. For example, please refer to FIG. 4 and FIG. 5, which are schematic diagrams of transmission coefficients and rejection bandwidths of the open stub 300 in different line widths. FIG. 4 shows curves of transmission coefficients, where curves TP_1˜TP_5 respectively indicate the line widths being 0.1 mm, 0.15 mm, 0.2 mm, 0.25 mm and 0.3 mm. FIG. 5 shows curves of transmission coefficients of a resonant point and rejection bandwidths, where curves TP_HM and BW_RJ respectively indicate the line widths being 0.1 mm, 0.15 mm, 0.2 mm, 0.25 mm and 0.3 mm. Therefore, as can be seen from FIG. 4 and FIG. 5, the effect of the open stub 300 reducing bandwidth is not sufficient. In other words, an ability of the micro-strip line open stub structure 30 filtering out noise is not sufficient around the RF band; thereby noise cannot be filtered effectively.